Document FR2094182 describes lubricants containing a rust-inhibiting and corrosion-inhibiting additive based on polyalkoxylated compounds, including a C18 amine. The quantity of amine used is very small. It is moreover indicated that the carbonate is the compound used to provide the basicity of the lubricant.
The marine oils used in low-speed two-stroke crosshead engines are of two types. On the one hand, cylinder oils ensuring the lubrication of the cylinder-piston assembly and, on the other hand, system oils ensuring the lubrication of all the moving parts apart from the cylinder-piston assembly. Within the cylinder-piston assembly, the combustion residues containing acid gases are in contact with the lubricating oil.
The acid gases are formed from the combustion of the fuel oils; these are in particular sulphur oxides (SO2, SO3), which are then hydrolyzed on contact with the moisture present in the combustion gases and/or in the oil. This hydrolysis generates sulphurous (HSO3) or sulphuric (H2SO4) acid.
To protect the surface of piston liners and avoid excessive corrosive wear, these acids must be neutralized, which is generally done by reaction with the basic sites included in the lubricant.
An oil's neutralization capacity is measured by its BN or Base Number, characterizing its basicity. It is measured according to standard ASTM D-2896 and is expressed as an equivalent by weight of potash per gram of oil or mg of KOH/g. The BN is a standard criterion making it possible to adjust the basicity of the cylinder oils to the sulphur content of the fuel oil used, in order to be able to neutralize all of the sulphur contained in the fuel, and capable of being converted into sulphuric acid by combustion and hydrolysis.
Thus, the higher the sulphur content of a fuel oil, the higher the BN of a marine oil needs to be. This is why marine oils with a BN varying from 5 to 100 mg KOH/g are found on the market. This basicity is provided by detergents that are overbased with insoluble metallic salts, in particular metallic carbonates. The detergents, mainly of an anionic type, are for example metallic soaps of a salicylate, phenate, sulphonate, carboxylate type etc. that form micelles where the insoluble metallic salt particles are maintained in suspension. The usual overbased detergents intrinsically have a BN conventionally comprised between 150 and 700 mg KOH per gram of detergent. Their percentage by mass in the lubricant is fixed as a function of the desired BN level.
Part of the BN can also be provided by non-overbased or “neutral” detergents with a BN typically less than 150. However, the production of formulae of marine engine cylinder lubricants where the entire BN is provided by “neutral” detergents cannot be envisaged: it would in fact be necessary to incorporate them in excessive quantities, which could be detrimental to other properties of the lubricant and would not be realistic from an economic point of view.
The insoluble metallic salts of the overbased detergents, for example calcium carbonate, therefore contribute significantly to the BN of the usual lubricants. It can be considered that approximately at least 50%, typically 75%, of the BN of the cylinder lubricants is thus provided by these insoluble salts.
The actual detergent part, or metallic soaps, found in both the neutral and overbased detergents, typically provides most of the remainder of the BN.
Environmental concerns have led, in certain areas and in particular coastal areas, to requirements relating to the limitation of the level of sulphur in the fuel oils used on ships.
Thus, MARPOL Annex 6 (Regulations for the Prevention of Air Pollution from Ships) issued by the IMO (International Maritime Organization) entered into force in May 2005. It sets a global cap of 4.5% m/m on the sulphur content of heavy fuel oils as well as creating sulphur oxide emission control areas, called SECAs (SOx Emission Control Areas). Ships entering these areas must use fuel oils with a maximum sulphur content of 1.5% m/m or any other alternative treatment intended to limit the SOx emissions in order to comply with the specified values. The % m/m rating denotes the percentage by mass of a compound relative to the total weight of fuel oil or lubricating composition in which it is contained.
More recently the MEPC (Marine Environment Protection Committee) met in April 2008 and approved proposed amendments to MARPOL Annex 6. These proposals are summarized in the table below. They present a scenario in which the restrictions on the maximum sulphur content become more severe with a worldwide maximum content reduced from 4.5% m/m to 3.5% m/m as from 2012. The SECAs (Sulphur Emission Control Areas) will become ECAs (Emission Control Areas) with an additional reduction in the maximum permissible sulphur content from 1.5% m/m to 1.0% m/m as from 2010 and the addition of new limits relating to contents of NOx and particles.
Current RegulationMARPOL Annex 6General LimitLimit for the SECAsMaximum Sulphur Content4.50% m/m1.50% m/m
Amendments to MARPOL Annex 6(MEPC Meeting No. 57 - April 2008)General LimitLimit for the ECAsMaximum Sulphur Content3.5% m/m on1% m/m onJan. 01, 2012Jan. 03, 20100.5% m/m on0.1% m/m onJan. 01, 2020Jan. 01, 2015
Ships sailing trans-continental routes already use several types of heavy fuel oil depending on local environmental constraints whilst making it possible for them to optimize their operating costs. This situation will continue whatever the final level of the maximum sulphur content permissible in fuel oils.
Thus provision is being made on most of the container ships currently under construction for the use of several bunker tanks, for a ‘high seas’ fuel oil with a high sulphur content on the one hand and for a ‘SECA’ fuel oil with a sulphur content less than or equal to 1.5% m/m on the other hand.
Changing between these two categories of fuel oil can require adaptation of the engine's operating conditions, in particular the use of appropriate cylinder lubricants.
At present, in the presence of fuel oil with a high sulphur content (3.5% m/m) and more), marine lubricants having a BN in the order of 70 are used.
In the presence of a fuel oil with a low sulphur content (1.5% m/m and less), marine lubricants having a BN in the order of 40 are used (this value is to be reduced in future).
In both these cases, a sufficient neutralization capacity is then achieved as the necessary concentration at basic sites provided by the overbased detergents of the marine lubricant is reached, but it is necessary to change lubricant each time the type of fuel oil is changed.
Furthermore, each of these lubricants has limits of use resulting from the following observations: the use of a 70 BN cylinder lubricant in the presence of a fuel oil with a low sulphur content (1.5% m/m) and less) and a fixed level of lubrication, creates a significant excess of basic sites (high BN) and a risk of destabilization of the unused overbased detergent micelles, which contain insoluble metallic salts. This destabilization results in the formation of deposits of insoluble metallic salts having a high degree of hardness (for example calcium carbonate), mainly on the piston junk, and can in the long term lead to a risk of excessive wear of a piston-liner buffing type.
Therefore, the optimization of the cylinder lubrication of a low-speed two-stroke engine then requires the selection of the lubricant with the BN appropriate to the fuel oil and to the engine's operating conditions. This optimization reduces the flexibility of operation of the engine and requires significant technical skill on the part of the crew in defining the conditions under which the change from one type of lubricant to the other must be carried out.
In order to simplify operations, it would therefore be desirable to have a single cylinder lubricant for two-stroke marine engine that can be used both with high-sulphur fuel oils and with low-sulphur fuel oils.
In particular, a need exists for formulations in which the BN is provided, alternatively to the overbased detergents, by compounds that do not give rise to metallic deposits when they are present in excess relative to the quantity of sulphuric acid to be neutralized.